Vacuum tight electromagnetic radiation permeable window seal



Nov. 10, 1964 R. J. BONDLEY 3,156,882

VACUUM TIGHT ELECTROMAGNETIC RADIATION PERMEABLE WINDOW SEAL Filed Aug. 22, 1962 z I/ I I I mvemor. Ra/p/z J. Bond/ey, by Q5.

His Afro/nay United States Patent 3,356,882 VAAIUUM TIGHT ELECTRUMAGNETIC RADIA- TllfiN PERMEABLE WINDDW SEAL Ralph E. Bondley, cotia, N.Y., assignor to General Electrio Jompany, a corporation of New York Filed Aug. 22, 1.962, Ser. No. 218,778 Claims. (6i. 333-98) This invention relates to a hermetic seal transparent to electromagnetic radiation or other types of radiation and particularly to such a seal securely joined to metal waveguide or gas tight structure.

Demands for increased power at microwave frequencies emphasize the necessity for using low dielectric loss ceramic, such as fused quartz, as a Window for passing radio frequency energy. The hermetically sealed ceramic Window material generally separates an electromagnetic Wave generating means from means utilizing the electromagnetic radiation. The window perrnits evacuation of the generating means while providing an exit therefrom for this output energy.

Unfortunately, it is difficult to join low expansion ceramic materials such as fused quartz in a permanent manner to a metal waveguide structure or the like. The ceramic window and metal waveguide combination must be capable of cycling through a large temperature range, not only in formation of the seal between the metal and ceramic but also during successive periods of operation. Fused quartz has a very low thermal coefficient of expansion, e.g. 5X10 per degrees C., much lower than any metal usable with ceramic material over wide temperature excursions. Since the quartz has a low mechanical strength in addition to its low thermal coefficient, a metal sealed to its outer periphery will pull away from the window as the temperature rises.

Efforts have been made to clamp the window within the supporting waveguide structure by means of an external annular ceramic clamping hoop, made of the same material as the window, and which is tightly sealed to the Waveguide at the plane of the window. Theoretically, the annular member, having the same coefficient of expansion as the window, will restrain expansion of the waveguide away from the window, thereby preventing the breaking of the seal between the window and waveguide. However, in practice the volumetric change of the frame or waveguide causes the seal between the hoop and waveguide to fail in tension during the cooling cycle. Moreover, the outer ceramic material usually fractures or cracks during the heating cycle.

It is therefore an object of the present invention to provide an improved vacuum tight seal capable of passing electromagnetic radiation and which is secure in construction, not being subject to failure during temperature cycling.

In accordance with an exemplary embodiment of my invention I provide a vacuum tight seal for passing electromagnetic radiation including a central low expansion ceramic material member or window housed in a metal frame waveguide around which is disposed a hoop of a second material. The central member is preferably quartz. The second material, a refractory composition material, e.g., a ceramic material, has a thermal coefficient of expansion which allows volumetric expansion of the metal frame or waveguide while confining the inner diameter of the metal frame or waveguide in contiguously adhering relation to the central member.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further objects and advantages thereof,

may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is an end view of a waveguide including a window and the external clamping hoop surrounding the waveguide and window, and

FIG. 2 is a cross-section taken at 22 in FIG. 1.

Referring to FIG. 1, a low dielectric loss ceramic central member or window 1 is firmly housed in a metal frame or waveguide 2 around which is disposed a hoop of refractory composition or ceramic material 3 securely engaging the waveguide and in juxtaposition with the window. Both the window and the hoop are secured to the window material with a sealing material such as, for example, indium, in the manner disclosed in my copending application Serial No. 741,713, filed June 11, 1958, entitled Composite Bodies and Method of Making, which is assigned to the assignee of the present invention.

It will be appreciated by those skilled in the art that during high temperature cycling of the structure shown, the metal frame 2 will tend to expand much more rapidly than the ceramic material 1 due to its much higher thermal coefficient of expansion. During such expansion the seal between window and frame will tend to break. In like manner, during cooling the metal frame 2 will tend to contract away from ceramic hoop 3', tending to break the seal therebetween, rendering useless the clamping action of hoop 3. Furthermore, during an increase in temperature, the expansion of frame 2 is likely to fracture the ceramic hoop 3.

In accordance with the present invention the metal frame 2 expands volumetrically, but its inner diameter is restrained to maintain contact with ceramic member 1. Viewing but a small segment of frame 2, the metal of the frame expands in volume due to the increase in temperature whereby the space it occupies is greater at the higher temperature. However, the material of frame 2 is restrained at its inner radius to the limited expan sion of the ceramic member 1. The metal members expansion is thus circumferentially quite limited around its inner radius and therefore does not leave ceramic member 1. This correctly proportioned yieldable restraint is accomplished by a hoop 3 having a predetermined coefficient of expansion, less than metal frame 2 but slightly more than ceramic member 1. The coefficient of expansion of hoop 3 just permits the volumetric expansion of material in frame 2 so that the frame does not fracture the hoop. Rather the hoop expands with the outer radius of the frame. But the volumetric expansion of frame 2, as inwardly confined, prevents a cleavage between the frame 2 and the central ceramic member.

The expansion characteristics for the hoop may be conveniently determined as follows:

The inner diameter of the hoop is:

Then the size change of the window and frame is equal to This is the amount bywhich the window and frame expand providing they are kept together, that is assuming there to beno separation between the ceramic Window and the metal frame.

The inner diameter of the hoop should change an equal amount, i.e.

As a practical example assume the window to be fused quartz 1 /2 inches in diameter and with a thermal expansion, K of 0.5 10 per C. The frame is silver waveguide and has an expansion coefficient, K equal to 18 10 per C., and a thickness equal to 0.012 inch. Substituting these values in Expression 4 above, and solving for K we find K should equal 0.78 10 per C. This is nearly the exact expansion coeificient for a glass sold under the name Vycor, i.e., Corning 790 glass. The constituency of this glass is predominantly silica, e.g. 96 percent silica. The residual 4 percent left in the glass during manufacture accounts for the increase in thermal coefiicient of expansion as compared with pure silica.

In constructing the vacuum tight seal according to the present invention, the low dielectric loss ceramic material is sealed in a close fitting Waveguide and may be sealed using indium solder as the sealing material in the manner as set forth in my aforementioned copending application Serial No. 741,713, filed June 11, 1958, and assigned to the assignee of the present invention. Briefly and by way of example, the edge of the central ceramic member is first painted with a fugitive adhesive such as polyisobutylene after which the edge is dusted with titanium hydride powder. Finely divided solder, e.g. indium, is then applied over the hydride after which the work area is evacuated and the temperature is raised to approximately 550 C. to dissociate the hydride. In a matter of from 3 to 5 minutes at this temperature, the metals will have reacted with thequartz. The silver waveguide or frame 3 is then alloyed with the primarily indium surface edge at a temperature of approximately 850 C. to form the seal therebetween. A close fitting hoop of ceramic material is fitted to snugly engage the, outer periphery of the waveguide in juxtaposition with a central window to exert a clamping action upon the combination of this waveguide and central window. The hoop is also sealed to the waveguide, for example, in the manner as set forth in my aforementioned copending application. As indicated, the ceramic or refractory composition clamping hoop is formed of a material having a coefficient of expansion appropriate for restraining the inner diameter of the waveguide to the limited expansion of the ceramic window, while allowing volumetric expansion of the waveguide material in substantially one direction, i.e. the radially outward direction, and may be conveniently determined in accordance with Expression 4.

The term electromagnets radiation above is intended in its broad sense to include forms-of radiation in addition to radio frequency, Wave energy, e.g. ultra-violet radiation.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made Without departing from my invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scopeof my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A vacuum tight seal transparent to electromagnetic radiation comprising a' central member of low dielectric loss ceramic material having a coeificient of expansion substantially less than the coefficients of expansion which characterize metals, a supporting structure comprising a metal frame immediately surrounding said central member, said frame therefore having a large coefficient of expansion as compared with the central member, and a refractory composition restraining member in clamping relation to said metal frame in juxtaposition with said central member, the restraining member having a coeflicient of expansion greater than said central memher but less than said frame and also less than coefficients of expansion which characterize metals, said coefficient being of a value to cause expansion of said restraining member with volumetric expansion of said frame while causing the inner diameter of said frame to follow the limited expansion of and minimize stress changes with temperature changes of said central member thereby preventing cleavage therefrom, said frame having a thickness which is small in comparison to the thickness of said restraining member.

2. A vacuum tight seal transparent to electromagnetic radiation comprising a quartz disc having a low coeificient of expansion substantially less than the ccefiicient of expansion which characterize metals, a supporting structure comprising a tubular metal frame immediately surrounding said disc and sealed thereto, said frame having a large coefficient of expansion as compared with the quartz material, and a ceramic restraining member in a clamping relation immediately surrounding said metal frame in juxtaposition with said disc, the ceramic restraining member having a coeflicient of expansion greater than said disc but less than said frame and also less than the coetficients of expansion which characterize metals, said coefficient being of a value to cause expansion of said restraining member with volumetric temperature expansion of said metal frame while causing the inner diameter of said metal frame to follow the limited expansion of said disc with change in temperature thereby minimizing stress changes with temperature change to prevent cleavage with said disc,-said frame having a thickness which is small in comparison to the thickness of said restraining member.

3. A vacuum tight seal transparent to electromagnetic radiation comprising a disc of fused quartz, a hollow circular thin metal waveguide concentrically sealed to said disc in supporting relation wherebysaid quartz disc closes off said waveguide, said waveguide having a large coefiicient of expansion as compared with the quartz disc, and an annular ceramic restraining hoop which is relatively thick compared to said waveguide and which is sealed to said waveguide in clamping relation immediately surrounding said Waveguide in juxtaposition with said disc, the ceramic annular restraininghoop having a coefficient of expansion greater than quartz but less than said Waveguide and of a value to cause expansion of said restraining hoop with volumetric temperature expansion of said waveguide while confining the inner diameter of said waveguide in contiguously adhering relation to said disc.

4. A vacuum tight seal transparent to electromagnetic radiation comprising a disc of fused quartz, a hollow circular thin metal waveguide concentrically sealed to said disc in supporting relation whereby said quartz disc closes off said waveguide, said waveguide having a large coelficient of expansion as compared with the quartz disc, and an annular glass restraining hoop in clamping relation immediately surrounding said waveguide in juxtaposition with said disc which hoop is relatively thick radially in comparison to the radial thickness of said waveguide, a sealing material intermediate said glass restraining hoop and said waveguide and also intermediate said waveguide and said quartz disc, the glass restraining hoop having a coefiicient of expansion greater than quartz but less than said waveguide and less than the coefiicients of expansion which characterize metals being of a value to cause expansion of said glass restraining hoop with volumetric temperature expansion of said Waveguide while causing the inner diameter of said waveguide to follow the limited expansion of said disc with change in temperature to prevent cleavage at the location of said sealing substance between said Waveguide and said disc.

5. A vacuum tight seal transparent to electromagnetic radiation comprising a disc of low dielectric loss ceramic material having a low coefficient of expansion substantially less than the coefficients of expansion which characterize metals, a supporting structure comprising a metal frame immediately surrounding said disc and sealed thereto, said frame having a large coeflicient of expansion as compared With the ceramic material, and a ceramic restraining hoop in a clamping relation immediately surrounding said metal frame in juxtaposition with said disc, the coefficient of expansion of said restraining hoop also being less than the coeificients of expansion which characterize metals and of a value to cause expansion of said restraining hoop allowing for volumetric expansion of said frame while causing the inner diameter of said frame to follow the limited expansion of said disc thereby minimizing stress changes with temperature change, the coen cient of expansion of said restraining hoop being equal to the product of the diameter of the disc times its coefficient of expansion added to the product of twice the thickness of the metal frame times its coeificient of expansion, all divided by the diameter of the disc added to twice the thickness of the metal frame, said frame having a thickness which is small in comparison to the thickness of said restraining hoop.

Refereuees Cited by the Examiner UNITED STATES PATENTS 3,058,074 10/62 Kane 333-98 HERMAN KARL SAALBACH, Primary Examiner. 

1. A VACUUM TIGHT SEAL TRANSPARENT TO ELECTROMAGNETIC RADIATION COMPRISING A CENTRAL MEMBER OF LOW DIELECTRIC LOSS CERAMIC MATERIAL HAVING A COEFFICIENT OF EXPANSION SUBSTANTIALLY LESS THAN THE COEFFICIENTS OF EXPANSION WHICH CHARACTERIZE METALS, A SUPPORTING STRUCTURE COMPRISING A METAL FRAME IMMEDIATELY SURROUNDING SAID CENTRAL MEMBER, SAID FRAME THEREFORE HAVING A LARGE COEFFICIENT OF EXPANSION AS COMPARED WITH THE CENTRAL MEMBER, AND A REFRACTORY COMPOSITION RESTRAINING MEMBER IN CLAMPING RELATION TO SAID METAL FRAME IN JUXTAPOSITION WITH SAID CENTRAL MEMBER, THE RESTRAINING MEMBER HAVING A COEFFICIENT OF EXPANSION GREATER THAN SAID CENTRAL MEMBER BUT LESS THAN SAID FRAME AND ALSO LESS THAN COEFFICIENTS OF EXPANSION WHICH CHARACTERIZE METALS, SAID COEFFICIENT BEING OF A VALUE TO CAUSE EXPANSION OF SAID RESTRAINING MEMBER WITH VOLUMETRIC EXPANSION OF SAID FRAME WHILE CAUSING THE INNER DIAMETER OF SAID FRAME TO FOLLOW THE LIMITED EXPANSION OF AND MINIMIZE STRESS CHANGES WITH TEMPERATURE CHANGES OF SAID CENTRAL MEMBER THEREBY PREVENTING CLEAVAGE THEREFROM, SAID FRAME HAVING A THICKNESS WHICH IS SMALL IN COMPARISON TO THE THICKNESS OF SAID RESTRAINING MEMBER. 